Applications and Case Studies > Acid mine/rock drainage

Modeling pit lake pH over time as inflows vary

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Gaulty:
SOLUTION_SPREAD
    -units    mg/l
  pH               Alkalinity      Cl    S(6)       Al       Ba      Ca             Mg        K      Na      Fe
       mg/l as Ca0.5(CO3)0.5    mg/l    mg/l     mg/l     mg/l    mg/l    mg/l charge     mg/l    mg/l        
 7.5                       85       7    1190    0.028    0.012     233            143     5.17    15.5    0.12
 2.6                               19    4000      119    0.003     370            375     0.76     8.8     178
 8.4                      179     1.7     616     0.02    0.005     207           62.4     3.14    2.69    0.01
 6.4                     70.5     0.1     3.1     0.15     0.01    24.2            3.9      0.6    1.09    0.31
 5.6                                      0.6                      0.17          0.025    0.017    0.02        
END
MIX 1
    1    79
    2    2.5
    3    6.5
    4    10
    5    2
USE mix 1
EQUILIBRIUM_PHASES 1 atmosphere
    Co2(g)    -2.5 10
    O2(g)     -0.68 10
    Fe(OH)3(a) 0 0
    Calcite   0 0
    Barite    0 0
    Gypsum    0 0
SAVE solution 6
END
MIX 2 2016
    2    2.575
    3    6
    4    10
    5    2
    6    80
USE mix 2
USE equilibrium_phases 1
Save solution 7
END
MIX 3 2017
    2    2.65
    3    5.5
    4    10
    5    2.05
    7    80
USE mix 3
USE equilibrium_phases 1
Save solution 8
END
MIX 4 2018
    2   2.725
    3   5
    4   10
    5   2.1
    8   81
USE mix 4
USE equilibrium_phases 1
Save solution 9
END
MIX 5 2019
    2    2.8
    3    4.5
    4    10
    5    2.15
    9    81
USE mix 5
Use equilibrium_phases 1
Save solution 10
END
MIX 6 2020
    2    2.875
    3    4
    4    10
    5    2.2
    10   81
Use Mix 6
Use equilibrium_phases 1
Save Solution 11
END

Hi there,
I am trying to model the pH of the mixed layer of a pit lake over time. The proportion of each inflow changes over time (as set up with MIX). I want to add each proportion of the inflow solution to the previous year mixed layer and allow it to equilibrate. Everything was going fine until I got to simulation  6 - I now get a convergence error.

OUTPUT:
------------------------------------
Reading input data for simulation 6.
------------------------------------

   MIX 5 2019
       2    2.8
       3    4.5
       4    10
       5    2.15
       9    81
   USE mix 5
   Use equilibrium_phases 1
   Save solution 10
   END
-----------------------------------------
Beginning of batch-reaction calculations.
-----------------------------------------

Reaction step 1.

WARNING: Maximum iterations exceeded, 100

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying smaller step size, pe step size 10, 5 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying reduced tolerance 1e-16 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying increased tolerance 1e-14 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying diagonal scaling ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying diagonal scaling and reduced tolerance 1e-16 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying scaling pure_phase columns 1e-10 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying scaling pure_phase columns and diagonal scale 1e-10 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying increased scaling 1e-09 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Skipping optimize equations for first 5 iterations ...

WARNING: Maximum iterations exceeded, 100

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Adding inequality to make concentrations greater than zero.

WARNING: Maximum iterations exceeded, 100

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying reduced tolerance 1e-17 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: Trying reduced tolerance 1e-18 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: The program has failed to converge to a numerical solution.

The following equations were not satisfied:
ERROR:               A(H2O) Activity of water has not converged.    Residual: 7.287599e-08

ERROR:           Fe(OH)3(a) Pure phase has not converged.    Residual: -6.355765e-05

ERROR:                O2(g) Pure phase has not converged.    Residual: -8.932042e-06


Sorry for the large copy of text.
I read on another forum that the activity of water error could be due to the file being so large but this does not seem correct. I have computed large files in phreeqc before and never had this issue. Any idea why this is occurring and how I can fix the problem?  I am hoping to the take simulation out another 50 years.

Many thanks,
Kristen


dlparkhurst:
Looks like you are mixing to a volume of 100 L. PHREEQC will work best if the volume is close to 1.

I cannot be sure what the composition of solution 5 is because the tabs are not preserved. I will take a look at it if you attach your file.

Gaulty:
David, thank you for your prompt response.

I have attached my input and output file.

In the meantime I will try updating the values so the solution is 1 L.

Many thanks again.

Gaulty:
David, your suggestion to update to 1 L seems to have resolved the issue! The file now runs without errors or warnings. :)

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